1. Field of the Invention
The present invention relates to optical-disc driving apparatuses, optical-disc driving methods, storage media, and programs. More particularly, the present invention relates to an optical-disc driving apparatus and an optical-disc driving method capable of reliably controlling an optical disc, a storage medium, and a program.
2. Description of the Related Art
A driving apparatus for driving an optical disc, which is a storage medium, irradiates an optical disc with a light beam for reading or writing. This light beam is ideally perpendicular to the surface of the optical disc.
However, if the optical disc has radial skew or the like, the angle between the surface of the optical disc and the optical axis of an objective lens in an optical pickup in the driving apparatus is shifted from perpendicular. As a result, the light beam is incident on the surface of the optical disc at an angle shifted from perpendicular, thus causing an aberration in the light beam.
With any aberration, when data is recorded on the optical disc, record marks are not appropriately formed on the optical disc. When data is read out from the optical disc, the signal-to-noise (S/N) ratio of a playback signal is degraded due to an increase in crosstalk or the like, thus causing a jitter.
As described above, if the optical disc has radial skew or the like, the function of the optical-disc driving apparatus is significantly damaged.
In order to correct an aberration due to a tilt of the optical axis, the objective lens is tilted in the radial direction to reduce in aberration and to inhibit an occurrence of a jitter of a playback signal.
A known driving apparatus having a tilt adjustment mechanism for tilting the objective lens in the radial direction includes, in its optical pickup body, a tilt sensor, such as an infrared sensor, for converting radial skew into a voltage. The driving apparatus is driven such that the optical axis of the objective lens in the optical pickup in the driving apparatus is perpendicular to the surface of the optical disc by driving the tilt adjustment mechanism based on the converted voltage.
When the optical disc is tilted with respect to the optical axis with the objective lens being in tracking control, a differential amplifier outputs signals corresponding to the tilt and the amplitudes of the pit signals are varied depending on the tilt. A known optical device detects a tilt without a tilt sensor by observing such signals by using a control circuit (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-291257). With this device, since a DC offset does not occur due to a tilt in a tracking servo control, the stability of a tracking control system against a tilt can be improved only by correcting a decrease in pit depth caused by the tilt by using a variable gain amplifier.
Furthermore, a known optical disc device detects a disc tilt or a lens tilt at which a push-pull signal of a photo-sensor has a maximum amplitude at an arbitrary position on the radius of the optical disc (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-141761). In the optical disc device, a light beam from a laser diode is converted into a parallel light beam by a collimator lens, the parallel light beam is reflected by a reflective mirror in the direction of an objective lens, the reflected light beam is focused on the optical disc through the objective lens, and the light beam reflected from the optical disc is incident on the photo-sensor through the collimator lens and a polarization splitter. In recording or playing back of a signal, an amount of tilt (tilt angle) of the disc or lens is set in accordance with the position on the radius.
However, when a tilt sensor, such as an infrared sensor, is provided, it is necessary to calibrate the sensitivity or an offset voltage in advance. Accordingly, there are problems in that the adjustment is complicated and a practical accuracy cannot be attained and in that it is necessary to provide an additional tilt sensor in the optical pickup, thus undesirably raising the cost.
Furthermore, when no additional infrared sensor is provided, there is a problem in that a complicated process, such as a process of detecting the amplitude of a signal, is required.